1. Field of the Invention
The present invention relates to a semiconductor device utilizing a semiconductor thin film and to techniques for fabricating such a semiconductor device and, more particularly, to TFTs (thin-film transistors) utilizing a crystalline film containing silicon.
The semiconductor device referred to herein embraces every kind of device that functions by making use of a semiconductor. That is, the semiconductor device includes electrooptical devices (such as liquid crystal displays) and electronic devices on which such electrooptical devices are packed, as well as active components such as TFTs and MOSFETs (IGFETs). To make a clear distinction between two kinds of semiconductor devices, they will be referred to as the semiconductor device and as the display device, respectively.
2. Description of the Prior Art
In recent years, techniques for fabricating semiconductor circuits with TFTs (thin-film transistors) formed on substrates have evolved rapidly. Especially, an active matrix display using a crystalline silicon film (polysilicon film) as a thin-film semiconductor and having a common substrate on which both peripheral circuits and a pixel matrix circuit are packed have reached a practical level.
Among various kinds of active matrix displays, the active matrix liquid crystal display (AMLCD) is being actively developed as a display device adapted for use in a notebook computer, projector, or other mobile device. According to the mode of operation, AMLCDs are classified into two major groups: transmissive LCD and reflective LCD.
Presently, high information content, bright liquid crystal displays are being urgently developed. Structures have been developed in which each pixel is as small as less than 30 xcexcm in square to meet the XGA standard (1024xc3x97768 pixels) or the SXGA standard (1280xc3x971024 pixels).
In the aforementioned AMLCD, a voltage applied to the liquid crystal layer changes the optical response characteristic, thus turning on and off light. An auxiliary capacitor is usually added to each pixel to compensate for leakage of electric charge held on the liquid crystal layer.
We have already disclosed a method of obtaining a crystalline silicon film by making use of a catalytic element (typified by nickel) for promoting crystallization, to form a crystalline film containing silicon (see Japanese Unexamined Patent Publication No. 130652/1995). In particular, regions crystallized by direct introduction of a catalytic element (hereinafter referred to as the vertically grown regions) and crystallized regions (hereinafter referred to as the laterally grown regions) around the regions in which the catalytic element has been introduced are formed.
However, almost all catalytic elements are metal elements and so if the catalytic elements are left after the crystallization, the reliability of the finished TFTs will be impaired.
It is an object of the present invention to provide a simplified process sequence for fabricating a semiconductor device, the process sequence including a process step for effectively removing or reducing the catalytic element.
A method of fabricating a semiconductor device in accordance with the present invention starts with maintaining a catalytic element in contact with the whole surface or parts of an amorphous silicon film containing silicon. Otherwise, the catalytic element is introduced in the amorphous silicon film. The catalytic element promotes crystallization of the silicon. Then, the amorphous silicon film is heat treated to crystallize those portions of the amorphous silicon film that should become active components. A chemical element selected from group VB (group 15) of the periodic table is introduced into regions adjacent to the portions becoming the active components and also into regions becoming the lower electrodes of auxiliary capacitors. Finally, a heat treatment is made to getter the catalytic element into the regions doped with the group VB (group 15) element.
Another method of fabricating a semiconductor device in accordance with the present invention starts with maintaining a catalytic element with the whole surface or parts of an amorphous silicon film containing silicon. Otherwise, the catalytic element is introduced into the amorphous silicon film. The catalytic element promotes crystallization of the silicon. Then, the amorphous silicon film is heat-treated to crystallize those portions of the amorphous silicon film that should become active components. A chemical element selected from group VB (group 15) of the periodic table is introduced into regions adjacent to the portions becoming the active components and also into regions becoming the lower electrodes of auxiliary capacitors. The regions containing group VB (group 15) element is heat-treated to getter the catalytic element into the regions doped with group VB (group 15) element. Finally, a heat treatment is performed in an ambient containing a halogen element to getter the catalytic element into the ambient.
A main object of the invention is to provide a simplified process sequence for gettering a catalytic element out of a crystalline film after an amorphous film containing silicon is crystallized using the catalytic element.
A method of gettering the catalytic element is now described briefly. One feature of the invention is to use the gettering effect of an element selected from group VB (group 15) of the periodic table. Another feature of the invention is to employ the combination of the gettering effect of the group VB (group 15) element and the gettering effect of a halogen element.
The typical examples of the above-described catalytic element include nickel (Ni), cobalt (Co), iron (Fe), palladium (Pd), platinum (Pt), copper (Cu), and gold (Au). Our experiment has shown that nickel is the optimum element.
Examples of the group VB (group 15) element that getters the catalytic element include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The element that exhibits the most conspicuous effect is phosphorus (P).
As a typical example, nickel is used as the catalytic element, and phosphorus is used as the gettering element belonging to group VB (group 15). A heat treatment around 600xc2x0 C. results in the phosphorus and nickel being stably bonded. At this time, they can assume bonded states Ni3P, Ni5P2, Ni2P, Ni3P2, Ni2P3, NiP2, and NiP3.
Examples of the halogen element that getters the catalytic element described above include fluorine (F), chlorine (Cl), and bromine (Br). Especially, where nickel is used as the catalytic element, chlorine is liberated as volatile nickel chloride into a gas phase.
Where nickel is used as the catalytic element that accelerates the crystallization of the amorphous film containing silicon as described above, the catalytic element in the crystalline film can be removed or reduced by the gettering effect of the group VB (group 15) element.
Furthermore, the catalytic element can be removed or reduced by the gettering effect either of a group VB (group 15) element or of a halogen element. Where both gettering effects are used, greater advantages can be obtained.
Other objects and features of the invention will appear in the course of the description thereof, which follows.